1. Field of the Invention
The present invention concerns a method for reshaping tubes and a device for carrying out this method.
2. Description of the Related Art
To produce tubes, especially tubes with relatively large diameters, it is known that open seam tubes can be shaped from rolled sheet or strip and then welded along the longitudinal edges. Steel tubes of this type are suitable for very high pressures and are characterized by relatively great precision with respect to their wall thicknesses and surface finish. They are used in special applications that require high load limits such as in power plants and in the petrochemical industry. A disadvantage of this method is that precision production of the tubes is extremely time-consuming and thus cost-intensive due to the required rolling to final dimension.
Another well-known method is hydroforming, in which a tube section that serves as the starting material is used to produce complex hollow structural components of a desired geometry by cold working without a heat treatment. As a rule, an external shaping tool with an internal shape corresponding to the desired geometry of the component is used. The application of high hydraulic pressure to the interior of the tube section has the effect of bringing the tube section into the desired shape. This hydroforming method is used to produce complex hollow structural components such as housings for pipeline fittings, for example, as described in the published international patent application WO 99/52,659.
Furthermore, it is known from DE-AS 1 081 856 that relatively thin-walled tubes can be easily expanded by internal hydraulic pressure for the purpose of sizing the tubes. In this method, an external forming tool is used, which consists of a series of slit sleeves. These sleeves constitute a forming die, which tightly surrounds the tube during the expansion process. In this known method, the production of expanded tubes requires expensive actuating mechanisms for each pair of half-sleeves of the forming die. Moreover, because the wall of the tube is relatively thin to begin with, the diameter of the tube can be expanded to only limited extent by this method. It is not possible to produce tubes with significantly larger diameters.